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Who Says I Have no Filter?

January 17th, 2017

Mysteries of the Y Capacitor, Revealed

I’m becoming more of a big electronics genius every day. Staying humble is not easy.

Yesterday I posted about the resurrection of my old HP 3300A function generator, and I mentioned the weird capacitors across the AC mains. Right where the three wires entered the box, there were two caps (in one package). One went from neutral to ground, and the other went from hot to ground.

One of these caps had shorted out, and that had killed the generator. I had to replace them. When a cap between hot and ground shorts out, you have a hot wire going straight to ground, and that is pretty worthless.

A commenter mentioned a different type of component which would look just like a dual capacitor. He said EMI (electromagnetic interference) filters resembled capacitors, and he said they had inductors inside them. I went back to my schematic to check, and I didn’t see any inductors, so I think all I had were caps.

I started reading up on this stuff. I could not figure out why anyone would need filters right before a big transformer and two big reservoir caps with diodes. The reservoir caps and diodes kill AC and turn it into DC, so I would think they would also kill any high frequencies in the line. My guesses are worth about what I charge for them, however.

After some Googling, I learned that what I have is a “type Y” setup. Caps like this are intended to keep interference from passing through the power connection. If it’s coming from outside, they keep it out. If it’s coming from inside your device, they keep it inside.

I can’t call the dead HP engineers who designed the machine, so I can’t ask what the big concern was. I was thinking maybe the AC wires, if not filtered at the entrance to the box, could act as antennas and shoot RF into other internal components, distorting the signal the machine provides. I don’t really know.

Here’s an important fact: it is well known that when capacitors in this configuration die, they can short, and then you can get shocked. This is bad, unless you view death as a postive outcome. You need special capacitors designated X or Y. These capacitors will resist things like power surges. Do they eventually die from old age and short anyway? Search me.

I know nothing about X capacitors, but the Y type come in two varieties: Y1 and Y2. Y1 capacitors are expensive industrial components, so people use Y2. You can find them on Ebay. I guess I’ll have to order some and replace the brand-new capacitors I just installed.

I am wondering if there is any point in putting caps like this on my guitar amps. I think the odds that the sound will improve are infinitesimal, and the chance of electric shock is appreciable, but it would still be a cool feature to brag about.

I have always assumed it was impossible to get a shock from a grounded metal box, but I am used to being proven wrong, so I may as well get the caps.

There are components made to divert power surges. I forget what they’re called. There is a list of “immortal mods” for guitar amplifiers, and it lists things you can do to an amp to make it resistant to failure. The power surge shunt things are on the list. You can look it up. I don’t know if they would work on Y caps. Haven’t checked.

I know this information is fascinating to everyone. Try not to get so engrossed you forget to look away from my blog and do whatever it is you’re being paid to do.

4 Responses to “Who Says I Have no Filter?”

  1. Andy-in-Japan Says:

    The tech talk is actually a bit hypnotic.

    And the part of my mind not reading, is thinking of what devices *I* can put a few of those modifications on….

  2. Terrapod Says:

    You are on the right track. The HP product line was heavily used when I was in the HF/VHF radio design labs in the 70’s pre digital era.

    The only digital item in the plant used to crank formulas and run chart data points was a big honking HP10 electronic calculator the size of a breadbox. The TI 50 hand held slide ruler had just appeared around then but I could not afford it.

    By definition there is stray RF from god knows how many pieces of equipment floating about, so the power cord between your device and the wall is basically a really good antenna. The caps at the power cord entry to the box were to ground any stray RF from outside going in and conversely, anything inside from going out. Usually there is a large capacity, maybe .33 Mf or larger if space allows, and also a .0001 ceramic disk across it, from each wire to ground. The .33 should be NP (non polarized) but I have seen folks using electrolytics or tantalum both of which are polarized (the latter known to catch fire when they fail). If there is a coil in the can, it is inline and has a ferrite core, that would buck RF as well, I think it is only caps. Make your own filter, there is no reall critical issue.

  3. Ernie Bailey Says:

    You sure did bring back some memories of my days assigned to a Calibration Lab when I was in the US Navy.
    I went to Navy school at Treasure Island, in San Francisco. Went to Electronics Technician School for 48 weeks and then to Lowry AFB in Denver for the Air Force Calibration school for test equipment.
    Then was assigned to a Destroyer Tender for duty in the Calibration Lab. Calibrated all test equipment; metres, function generators, counters , multimeters, and maintained the equipment for calibrating time standards from wwv transmitters in Colorado.
    I know or remember nothing about your piece of equipment, good luck with it.

  4. Steve H. Says:

    Thanks for the interesting comments.

    Terrapod, it’s kind of unusual to see someone on the Internet comment on something they actually know about. Your comment is like an ivory-billed woodpecker sighting.

    Your comment about not being able to afford an HP calculator reminds me of the brochure someone showed me when I was in the 9th grade. He was talking about how he dreamed of having this or that model. I don’t recall the specifics, but it was probably something that cost hundreds of dollars and had exotic features such as a cosine key.